diplomatic workers have been ordered to leave the country this morning, as the fbi and other agencies try to figure out who carried out the malicious attack that killed ambassador chris stevens and three other americans. >> i want to assure you, we will bring their killers to justice. >> reporter: the attack was complex and preplanned. likely, for the anniversary of september 11th. it was around 10:00 at night in libya. 20 heavily armed attackers began firing at the libyan security guards protecting the american consulate, before setting the main building ablaze. inside, ambassador stevens and several others were trapped between the flames and the gunfire. stevens became separated from the others by the heavy, black smoke, while trying to exit the building. it was nearly a five-hour gun battle. additional security eventually regained some control, while employees escaped. but the buildings are now abandoned. it was also nearly 12 hours before ambassador stevens' body was returned by local libyans, who had carried him to a hospital in an effort to save him. a contingent of some 50 u.s.